MacDonald et al. SFG 30th October 2014
03/11/2014
Phase 1: Characterising the floodplain in 3D
Measuring the contribution of groundwater and soil water to flooding Alan MacDonald, Nicole Archer, Brighid Ó Dochartaigh
Funding:
Scottish Government SEPA, BGS, U Dundee, U W Australia
SFG: The Eddleston Water Project 30 Oct 2014
© NERC All rights reserved
Phase 2: Dynamic monitoring
Source: http://www.bgs.ac.uk/research/groundwater/catchment/Eddleston/home.html
© NERC All rights reserved
Hill slope soil permeability
Measure soil K in paired grid areas within the hillslope and floodplain of grassland and forest areas
Source: http://www.bgs.ac.uk/research/groundwater/catchment/Eddleston/home.html
DW1: 500 yr old deciduous forest
DW2: 180 yr old deciduous forest
CW3: Conifer forest 45 yr old
FW4: 180 yr old Wetland woodland
Source: Archer et al. 2013
http://dx.doi.org/10.1016/j.jhydrol.2013.05.043
© NERC All rights reserved
© NERC All rights reserved
Groundwater ‐ Baseline
Infiltration capacity
River losing water to gw
Old broadleaf woodland have 10‐15 times higher permeability than soils under neighbouring coniferous forest and pasture land. Dominant gw flow along valley
Some older anoxic confined gw
Chemistry of shallow gw similar to river
Soil K also related to parent geology
Soils on flood plain: low permeability
Runoff from a 1 in 10 rainfall event
Source: http://www.bgs.ac.uk/research/groundwater/catchment/Eddleston/home.html
Source: Archer et al. 2013
http://dx.doi.org/10.1016/j.jhydrol.2013.05.043
© NERC All rights reserved
British Geological Survey (c) NERC 2014
© NERC All rights reserved
1
MacDonald et al. SFG 30th October 2014
03/11/2014
Floodplain groundwater: baseline groundwater closely follows river and rainfall
© NERC All rights reserved
© NERC All rights reserved
Flood plain groundwater: flood events Groundwater levels close to hillslope
continue to rise for up to 5 days
Rainfall
Rising river levels propagate pressure effect
within 2 hours
Groundwater at floodplain edge
Groundwater near river
Fastest, greatest groundwater level response where confined
River
Groundwater levels close to river start to recede quickly after river level falls
© NERC All rights reserved
Interesting things in the detail …
© NERC All rights reserved
Groundwater and hillslope processes
•
•
•
Hillslope inflow causes slower & longer lasting groundwater level rise
Floodplain groundwater buffers hillslope runoff
Strongly influenced by antecedent conditions. Dry antecedent: impacts close to hillslope; wet antecedent: impacts across floodplain, including on river
Unsaturated antecedent conditions
•
•
•
© NERC All rights reserved
British Geological Survey (c) NERC 2014
Available storage for excess rainfall in hillslope
Small inflow from hillslope to floodplain
River still generally losing to groundwater Saturated antecedent conditions
•
•
•
Large inflow from hillslope to floodplain
Artesian groundwater: strongest close to hillslope
Groundwater discharge to river once river level recedes
© NERC All rights reserved
2
MacDonald et al. SFG 30th October 2014
Lessons learned
03/11/2014
Still learning !
1. There is a strong relationship between soil permeability and land use – established deciduous forests most permeable
2. Groundwater across most of the floodplain, except near its edges, is more closely coupled to river flow than local rainfall
3. Groundwater at the edge of the floodplain is weakly coupled to river flows, but strongly connected to rainfall infiltrating on adjacent hillslopes
4. The combination of soil water storage on hillslopes and groundwater storage in floodplain aquifers acts as an important buffer to flooding
© NERC All rights reserved
British Geological Survey (c) NERC 2014
© NERC All rights reserved
3